In recent years, portable electronic devices such as mobile phones and PDAs (Personal Digital Assistants) are in wide use. In a display section of a portable electronic device, a liquid crystal display device is frequently used because of its advantages in terms of thinness, light weight, and low power consumption.
In a liquid crystal display device, the display element itself does not emit light, unlike self-light-emitting type display devices such as CRTs and PDPs (plasma display panels). Therefore, in a transmission-type liquid crystal display device, an illuminator called a backlight is provided at the rear face side of the liquid crystal display element, and an image is displayed as the transmitted amount of the illumination light from this backlight is controlled by the liquid crystal display element in a pixel-by-pixel manner.
Liquid crystal display devices of various methods are known. However, some methods (e.g., methods using a TN type or STN type liquid crystal display element) have a disadvantage of narrow viewing angles, and various techniques are under development for overcoming this disadvantage.
As a representative technique for improving the viewing angle characteristics of a liquid crystal display device, there is a method of adding an optical compensation plate. There is also known a method of enhancing the directivity (degree of parallelism) of light which is emitted from a backlight before the light enters a liquid crystal display element, and allowing the light having traveled through the liquid crystal display element to be diffused by a light diffuser (e.g., Patent Document 1).
An example of a liquid crystal display device having a light diffuser is shown in FIG. 21. The liquid crystal display device 500 shown in FIG. 21 includes a liquid crystal display panel 520, a backlight 510 disposed at the rear face side of the liquid crystal display panel 520, and a light diffuser 530 disposed at the viewer's side of the liquid crystal display panel 520.
The liquid crystal display panel 520 includes a pair of substrates 521 and 522 and a liquid crystal layer 523 provided therebetween. Although not shown here, on the surfaces of the substrates 521 and 522 facing the liquid crystal layer 523, electrodes for applying voltages across the liquid crystal layer 523 and alignment films for defining the orientation directions of liquid crystal molecules contained in the liquid crystal layer 523 are formed.
The backlight 510 includes a light source 501 and a light guide plate 502 for guiding the light having been emitted from the light source 501 to the liquid crystal display panel 520. The light guide plate 502 has a front face (light-outgoing surface) 502a through which light goes out toward the liquid crystal display panel 520 and a rear face 502b opposing the light-outgoing surface 502a. A plurality of prisms 503 are provided on the rear face 502b of the light guide plate 502.
While propagating within the light guide plate 502, the light having been emitted from the light source 501 is reflected toward the liquid crystal display panel 520 by the prisms 503 formed on the rear face 502b, so as to go out through the light-outgoing surface 502a. Each prism 503 has two slopes that are slanted at respectively difference predetermined angles with respect to the light-outgoing surface 502a, so that the light which is emitted from the backlight 510 has a very strong intensity along the display surface normal direction (frontal direction). In other words, a high directivity is imparted to the light emitted from the backlight 510.
When the light emitted from the backlight 510 has a high directivity, the light traveling through the liquid crystal layer 523 can be uniformly modulated (i.e., a uniform retardation can be imparted to the light traveling through the liquid crystal layer 523). Therefore, the viewing angle dependence of display quality associated with the refractive index anisotropy of liquid crystal molecules can be reduced. As it is, the light having traveled through the liquid crystal layer 523 has a high directivity, and has a large imbalance in luminance (that is, a very high luminance exists along the display surface normal direction whereas luminances along oblique directions are low). However, through diffusion by the light diffuser 530, the luminance imbalance is reduced, and the viewing angle is broadened. As a result, the liquid crystal display device 500 is able to perform good displaying in a broad viewing angle range.
As the light diffuser 530, a prism sheet as shown in FIG. 22 can be suitably used, for example. The prism sheet 530 includes a plurality of prisms 531 of triangular prism shapes. Each prism 531 has slopes 531s which are slanted with respect to the display surface normal direction, such that the light going out from the liquid crystal display panel 520 is diffused by experiencing total reflection (or refraction) at the slopes 531s of the prism 531. By appropriately setting the tilting angle of the slopes 531s in accordance with the luminance distribution of the light emitted from the backlight 510, a predetermined luminance distribution can be realized in the light which goes out from the prism sheet 530.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 9-22011